| Background:Dengue fever is an acute infectious disease caused by the Dengue virus and transmitted by Aedes mosquitoes, and is now the most rapidly spreading mosquito-borne viral disease in the world. Infection with dengue virus can cause dengue fever, dengue haemorrhagic fever and dengue shock syndrome. Dengue is endemic or epidemic in more than 100 countries and regions in Asia, Oceania, America and Africa, and it is especially prevalent in Southeast Asia, west Pacific Ocean regions and southern Africa. The World Health Organization (WHO) has estimated that about 2.5 billion people worldwide are under the risk of Dengue virus infection,50 million people are infected by the Dengue virus each year, and among them, half a million suffer Dengue Hemorrhagic Fever (DHF).After the first report of dengue outbreak in Philadelphia America in 1780, the frequency of DF outbreaks and epidemic in the world had increased. Meanwhile, the morbidity of DF was increased 30 times in the past 50 years. With the impact of the greenhouse effect, international trade, the expanding distribution of mosquito vector, urbanization and the population, the epidemic interval of DF has become shorter and the epidemic area has expanded extensively. Considering the morbidity and mortality, DF is the second in the list of arthropod borne disease in the world.In mainland china, the character of dengue epidemic was fierce and the transmission was fast before the 1990s; afterwords, although the number of new cases has-become weakened, but the epidemic area expanded highly. DF was reported recently from Guangdong, Hainan, Fujian, Zhejiang and Jiangsu. DF mainly emerged as an outbreak in Guangdong and the outcome was most serious. The first outbreak of DF in China was reported in Guangdong Province in 1978. Since then, DF outbreaks have occurred in four provinces in southern China. Between 1990 and 2000, more than 10,000 DF cases were reported in Guangzhou alone, in Guangdong province. In 2014, more than 30,000 dengue cases and 4 death was reported in Guangzhou. In recent years, the epidemic interval of DF in southern China has become shorter and the epidemic area expanded enormously.The 2014 dengue fever epidemic in Guangdong PRC was the largest ever recorded in this province. The total number of cases till December 31,2014, reached 45,203, two-fold higher than any previous report. Cases were detected in all provincial cities except Meizhou, with a majority of 37,354 cases, in the provincial capital, Guangzhou.Urbanization is a global trend that results from economic development. It has a great impact on the human life and environment health, and the impact of urbanization on the occurrence and spread of disease has raised people’s wide concern. Higher speed urbanization has happened in China, and Guangzhou is a typical area of urbanization, in recent 10 years, the epidemic interval of DF has become shorter. In depth knowledge of various aspects of urbanization and delineating the mechanism of increased frequencies of dengue outbreaks is important to make prevention and control strategies.Urbanization refers to the increasing population of urban areas. Urbanization predominantly results in the physical growth of urban areas, leading to environmental changes. Ae. albopictus originated at the edges of forests and bred in natural habitats (tree holes, bamboo stumps, and bromeliads) and previously was considered to be a rural vector. However, this species has adapted well to suburban and urban environments with its larvae now breeding in artificial containers (tires, cemetery urns, and water storage containers), and has become the most important and sometimes the sole vector in urban areas (southern China and Italy (Ae. albopictus feeds aggressively and opportunistically during the day time on a wide range of hosts, dependent on their availability and the environment. When offered a choice, this species prefers human (anthropophilic behavior), but it also can feed on a large variety of animals such as cows, goats, dogs, birds, reptiles and amphibians (zoophilic behavior). The opportunistic zoophily enhances the spectrum of pathogens it can spread and the ecological niches it can occupy. Most literature refers to this species as resting and feeding outdoors (exophilic and exophagous), but recent reports provide evidence of geographical variation in this behavior with gravid females captured indoors in Rome, Italy. Whether this variation is dependent on different capture methodologies needs further investigation, because it is difficult to find Ae. albopictus resting in the wild.Environment changes like altitude cariation, air temperature, water temperature, illumination intensity and geographical region have a great impact on the ecology of mosquito species. Many problems have emerged as a result of urbanization, including environmental pollution, crowding, and the destruction of natural ecology. There are great difference among different urbanized areas, like temperature, vegetation and transportation. The habitat of Ae. albopictus also greatly affected by these factors. Guangzhou is the largest city of Guangdong province with a population of 12 million according to the 2007 census survey. Guangzhou is located in the southern point of Guangdong province, about 200 km from Hong Kong. The annual average temperature is 21.6℃, and rainfall is 1,983 mm. This climate is ideal for the development and reproduction of Ae. albopictus mosquitoes. The city has experienced rapid expansion as part of the economic development in the region in the past 3 decades. Human population density increased from 847 person/km2 to 1,337 person/km2 between 1990 and 2000. Also, nine major Dengue outbreaks have occurred in this city since 1980. Because Ae. albopictus is the sole Dengue vector and rapid urbanization is occurring in this city, Guangzhou is the ideal area to study how Ae. albopictus adapts to urban environments and how the Dengue virus is transmitted in the urban environment. As the Ae. albopictus vector is invading urban areas in Europe (e.g., Italy and France) and the USA, knowledge about its adaptation to the urban environment in Guangzhou would shed light on the potential of this important vector to become established in other parts of world, and thus on the vulnerability to Dengue outbreak.In the absence of specific antiviral therapy, control of transmission of DENV by vector management is the sole method available for reducing dengue-associated morbidity. So vector control remains the only option to reduce or prevent DENV transmission. Adult control depends largely on the use of insecticides. However, resistance to insecticides has been rising globally. Unfortunately, the extensive use of pesticides or further usage was not regulated in the world, which can lead to a reduction of the efficacy of larvicide or adulticide-based control programs. Aedes albopictus also is subjected to induced insecticide resistance, like the Anopheles mosquitoes already had a high resistance to deltamethrin in Africa due to the extensive use of insecticide. Further, many reports showed that, the insecticide resistance of different strains of mosquitoes are different in the same areas. It has been reported that, induced insecticide resistance emerge due to different insecticide selection pressure caused by usage of many insecticide simultaneously. It is important to understand the insecticide resistance statue and its involved mechanism in Ae. albopictus, the sole dengue vector in china. Although it is reported that, Ae. albopictus has a low insecticide resistance in Guangdong china, but the usage of insecticide was increased because of the increased frequency of DF outbreak in recent years. So we need to evaluate the efficacy of the control measure, which is an important step for developing and implementing appropriate strategies to control mosquito vector populations.Aedes albopictus is the major transmission vector of dengue in China. It can provide the basis for the prevention and control of DF that involves surveying the number and monitoring the density of Aedes albopictus. Dengue vector surveillance mainly including, Aedes albopictus adult mosquito density monitoring and larval density monitoring. Currently, the common monitoring methods of Aedes albopictus in our country are human-bait, UV light trapping, ovitrap oviposition and black-box method. It is evident that, these methods are inadequateand require to find a more sensitive and reliable monitoring method, that can reflect the nature of Aedes albopictus mosquito density and change in population dynamics. In recent years, BG-Sentinel traps (BGS trap) (BioGents Corporation, Regensbourg, Germany) have been used for collecting Aedes (Stegomyia) species such as Ae. aegypti, Ae. albopictus, and Ae. polinesiensis. BGS trap can be used with a variety of mosquito attractants (e.g., CO2, BG-lure, or octenol), thereby making it a versatile tool for mosquito community research and surveillance. Nevertheless, no studies have been reported of using this novel traps in China. This gap in knowledge will prevent us to utilize the BGS trap in mosquito surveillance and vector control in China. In this paper, we also evaluated the efficacy of BGS trap, against CDC light trap and Mosq-ovitrap, in terms of captures of mosquito species in the urban and suburban areas of Guangzhou, Guangdong province, China.Objective:Despite the fact that Dengue causes a major public health problem, the mechanisms for increased frequencies and magnitude of Dengue epidemics in southern China are not clear. Because Dengue epidemics coincide with the rapid urbanization in China as a result of economic development, it has been hypothesized that urbanization creates favorable conditions for the survival and reproduction of Dengue vectors and for the transmission of Dengue virus (DENV). Therefore, the present proposal will test the central hypothesis that environmental changes resulting from urbanization affect the biology and vectorial capacity of Aedes albopictus, the major vector of DENV in southern China. Exploring the mechanisms of Dengue epidemics will help us to develop and implement an effective Dengue prevention and control strategy.Methods:Our research contains three major parts:(1) Impact of environmental changes from urbanization on the ecology of dengue vector Aedes albopictus in Guangzhou, china. (2) Impact of the different usage of insecticide from urbanization on the insecticide resistant of dengue vector Aedes albopictus in Guangzhou, china. (3) Efficacy of the BG-Sentinel traps for mosquito collection in Guangzhou, China.In the first part, evaluating the impact of environment change from urbanization on the ecology of dengue vector Aedes albopictus in Guangzhou, china, we included four main aspects:1) The habitat types, abundance and seasonal dynamic in different settings (urban, suburban, and rural) in the Great Guangzhou area, China; 2) Adult Aedes albopictus density in different settings (urban, suburban, and rural); 3) The development time and emergence rate of Aedes albopictus in different settings (urban, suburban, and rural); 4) The life table analysis of Aedes albopictus in different settings (urban, suburban, and rural).Secondly, for evaluating the impact of the different usage of insecticide from urbanization on the insecticide resistant of dengue vector Aedes albopictus in Guangzhou, china, we focused on:1) Resistance status of larvae of Aedes albopictus in different settings (urban, suburban, and rural); 2) Resistance status of adult Aedes albopictus in different settings (urban, suburban, and rural); 3) Exploring the insecticide resistance mechanism of adult Aedes albopictus, consisting of the activity of detoxicate enzyme and resistant gene; 4) the investigation of commonly used insecticide in different settings (urban, suburban, and rural).Finally, to evaluate the efficacy of the BG-Sentinel traps for mosquito collection in Guangzhou, China, we followed:1) The effectiveness of three kinds of traps to catch different species of mosquitoes in lab conditions; 2) The effectiveness of three kinds of traps to catch different species of mosquitoes in the field; 3) Surveillance of the population dynamic of the mosquitoes by using three kinds of traps in the field. 4) The blood-fed status of the mosquitoes in the field surveillance by the BG-Sentinel traps and CDC light traps.Results:(1) During our survey between May and November in 2013, immature Ae. albopictus were most often found in abandoned tires, flower pots, plastic bucket, disposable food tins and gutter in urban areas. In suburban areas, Ae. albopictus larvae were common in abandoned tires and clay pots, plastic bucket, disposable food tins and plastic basin. Whereas in rural areas, Ae. albopictus larvae were frequently found in plastic buckets, plastic basins, clay pots, disposable food tins and building tool. During our survey period, we found 2639,2523, and 1760 aquatic habitats in urban, suburban, and rural areas, respectively. By using the BG Sentinal Trap, the monthly average density of Ae. albopictus adults was significantly higher in urban areas than that in suburban and rural areas, and it was significantly higher in suburban area than that in rural areas. In the natural habitat, Ae. albopictus adult emergence rates was the highest in the urban area than suburban and rural area, with the lowest rate in the rural area. Larval development time in urban areas was significantly shorter than that in both the suburban and rural areas. The life span of adult mosquito was significantly longer in urban areas than that in suburban and suburban longer than rural area.(2) During our survey between June and October in 2014, Larval bioassays resulted in low resistant ratios (RRs) indicating that all of the six Guangzhou populations of Ae. albopictus were sensitive to the larvicides Bti. Further, all the populations had values of RRs for propoxur that excluded 1, ranging from 1.60 (RDS) to 3.74 fold (UYS). All the populations had values of RRs for Malathion that included 1, ranging from 0.74 (SPJ) to 1.94 fold (UYS). However, all the populations showed resistance to Deltamethrin with RRs of 11 to 38 fold, the average RR50 in urban, suburban and rural area compare to SSS is 27,14 and 18 fold, respectively. The KDT for Ae. albopictus exposed to Propoxur indicated that most KDT50 values from field populations were lower than those of the SSS, except for UYS and RCD that showed higher KDT50. For DDT, all the populations were resistant (UBT, UYS, RCD) or resistance is suspected (SBL, SPX and RPL). For deltamethrin, all the populations were resistant (UYS) or resistance is suspected (UBT, SBL, SPX and RPL) except for RCD. For Malathion, all the populations were still susceptible except for UYS (resistance is suspected, mortality is 97%). For Propoxur, all the populations were still susceptible except for UYS (resistant, mortality is 80%) and RCD (resistance is suspected, mortality is 98%). In summary, the populations from urban areas (UBT and UYS) have the lowest mortality rate (77% and 89.5%) to both DDT and deltamethrin than suburban (91.5% and 96%) and rural areas (92% and 98%). For the survey, we found that the usage frequency of pyrethrum (cypermethrin) in urban community is higher than suburban and rural areas.(3) In laboratory release-recapture experiment, BG-Sentinel traps caught significantly more mosquitoes than the CDC light traps and Mosq-ovitraps except the Anopheles sinensis. And the BG-Sentinel traps also had a higher efficacy in catching female and male Aedes albopictus and Culex quinquefasciatus than the CDC light traps and Mosq-ovitraps in the lab. In the field trial, BG-Sentinel traps collected more Aedes albopictus than the CDC light traps and Mosq-ovitraps both in urban and suburban areas. And BG-Sentinel traps had the similar efficacy in catching Culex quinquefasciatus in urban and suburban areas. Furthermore, BG-Sentinel traps are more sensitive to surveillance of the population density of Aedes albopictus than the CDC light traps and Mosq-ovitraps during the peak months of the year 2013. BG-Sentinel traps can collect more blood fed mosquitoes than the CDC light traps in both urban and suburban in Guangzhou, China.Conclusions:Environment changes from urbanization had a great impact on the ecology of dengue vector, Aedes albopictus in Guangzhou, china. A large variation in the use of insecticides from urbanization also increased the select pressure of this vector in this area. Urbanization substantially increased the density, larval development rate, and adult survival time and insecticide resistance of Ae. albopictus, which in turn potentially increased the vector capacity, and therefore, disease transmissibility. Mosquito ecology and its correlation with dengue virus transmission should be investigated in different habitats and should be compared in different environmental settings to implement an effective infection control strategy. |
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